Pneumatic radial tire with specified carcass strength coefficient

ABSTRACT

A pneumatic radial tire wherein both end portions of at least one layer of a carcass layer are turned up around right and left bead cores from the inside to the outside of the tire, and at least two layers of belt layers formed by arranging reinforcing cords in such a fashion that their inclining directions with respect to a tire circumferential direction cross one another in mutually opposite directions between the layers are disposed on the outer circumferential side of the carcass layer of a tread portion. A carcass strength coefficient K defined by the formula (1) in the specification is 0.15 to 0.35 N/mm·kPa at portions on the tread portion center side of positions P which are away by 10% of a belt width of a belt layer having the second greatest width from both edges thereof towards its inside, and is 0.5 N/mm·kPa at portions from the positions P to the bead portions.

TECHNICAL FIELD

This invention relates to a pneumatic radial tire. More particularly,this invention relates to a pneumatic radial tire which can reduce aproduction cost while keeping tire performance at the same level as thatof prior art tires, and particularly to a pneumatic radial tire suitablefor passenger cars.

BACKGROUND ART

Generally, pneumatic radial tires are equipped with at least one carcasslayer formed by arranging reinforcing cords in a tire radial directioninside a tire as a tire skeletal structure, and this carcass layer keepsa tire internal pressure. At least two belt layers, each of which isformed by arranging reinforcing cords in such a fashion that theirinclining directions with respect to a tire circumferential directioncross one another in mutually opposite directions between the layers,are disposed on the outer circumferential side of the carcass layer of atire tread portion. These belt layers protect the carcass layer from animpact from a road surface and an external damage, and function as a“hoop” keeping rigidity in the tire circumferential direction.

Recently, price competition has become severer and severer in the tireindustry, too, and the pneumatic radial tire described above must beproduced essentially at a low cost without spoiling conventional tireperformance.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide apneumatic radial tire which can attain a lower cost of production whilekeeping tire performance at the same level as that of conventionalpneumatic radial tires.

The pneumatic radial tire according to the present invention foraccomplishing the object described above comprises at least one carcasslayer formed by arranging reinforcing cords, both end portions of whichare turned up from the inside to the outside of the tire round right andleft bead cores, respectively, and at least two belt layers formed byarranging reinforcing cords in such a manner that their incliningdirections with respect to a tire circumferential direction cross oneanother in mutually opposite directions between the layers and disposedon the outer circumferential side of the carcass layer of a treadportion, characterized in that a carcass strength coefficient K of thecarcass layer defined by the following formula (1) is set so as tosatisfy a value of 0.15 to 0.35 N/mm·kPa at portions on the treadportion center side of positions P which are away by 10% of a belt widthof a belt layer having the second greatest width from edges thereoftowards its inside, and a value of at least 0.5 N/mm·kPa at portionsranging from the positions P to bead portions:

K (N/mm·kPa)=[number of reinforcing cords arranged (pcs/mm)]×[strengthof reinforcing cords (N)]×[number of carcass layers]÷[maximum airpressure (kPa)]  (1)

The inventors of the present invention have carried out intensivestudies on radial tire structures whose strength design is made on thebasis of a maximum air pressure, and have specifically noted thatbecause the belt layers are disposed at the tread portion, the tireinternal pressure is kept by the carcass layer and the belt layers inwhich they bear their share of that. Therefore, the present inventorshave realized that the strength of the carcass layer can be reduced atleast in the area of the tread portion and a part of the tire internalpressure can be shared by the belt layers.

In other words, in the pneumatic radial tires according to the priorart, the carcass layer must bear the internal pressure at sidewallportions, too, so that the carcass strength has been designed to thestrength necessary for reinforcing the sidewall portions. At the treadportion, however, the belt layers bear a part of the tire internalpressure and for this reason, the carcass strength has been greater thannecessary in the conventional pneumatic radial tires.

Besides the observation described above, the present inventors have paidspecific attention to the carcass strength coefficient K which isdefined by the formula (1) and falls within substantially the same rangeeven when the kinds of the tires are different. As to the two layers ofthe belt layers formed by so arranging the reinforcing cords as to crossone another, therefore, the present inventors have found that when thecarcass strength coefficient K is calculated, the belt layers can bear astrength corresponding to 0.35 N/mm·kPa such that the total of the beltstrength coefficient and carcass strength coefficient is at least 0.5N/mm·kPa. Accordingly, in the carcass layer at the tread portion wherethe belt layers bear a part of the tire internal pressure, the carcassstrength coefficient K need not be set to 0.5 n/mm·kPa or more as hasbeen required in the prior art tires, and it has been found out thattire durability can be secured sufficiently even when the value islowered to the minimum value of 0.15 N/mm·kPa by reducing the number ofarrangement of the reinforcing cords (end number), etc. It has beenfound out also that in order to improve the production cost, the upperlimit of the carcass strength coefficient K must be set to 0.35N/mm·kPa. Further preferably, the coefficient K is within the range of0.2 to 0.3 N/mm·kPa.

Therefore, in the sidewall areas where a part of the tire internalpressure cannot be borne by the belt layers, the carcass strengthcoefficient K is set to at least 0.5 N/mm·kPa in the same way as in theprior art tires. The upper limit of this carcass strength coefficient Kis preferably 1.5 N/mm·kPa. Even when the carcass strength coefficient Kis increased beyond this upper limit, the carcass strength becomesexcessive as far as tires for passenger cars in general are concernedand such a strength is meaningless but merely increases the tire weight.However, the carcass strength coefficient K must not be decreased below0.15 N/mm·kPa in the entire area occupied by the belt layers, and thevalue of the carcass strength coefficient K of at least 0.5 N/mm·kPamust be maintained at portions up to positions P which are away by atleast 10% of the belt width from the edges of the belt layer having thesecond greatest width toward inside, in the same way as in the sidewallareas. On the other hand, the area in which the carcass strengthcoefficient K is set to 0.15 to 0.35 N/mm·kPa at the tread portion asdescribed above need not be the area corresponding to the belt layers,but may be the area on the tire center side of the positions P describedabove.

As described above, the carcass strength coefficient is smaller than inthe prior art tires at the tread portion at which the belt layers bear apart of the tire internal pressure and consequently, the number ofarranged reinforcing cords of the carcass layer, the diameter of thereinforcing cords, the number of the carcass layers, etc, can bereduced. Therefore, the material cost can be reduced, and the reductionof the production cost as well as the tire weight can be accomplished.

Moreover, even when the materials of the carcass layer are reduced asdescribed above, carcass durability required at the tread portion can besecured, and the drop of tire performance does not occur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a tire meridian half-sectional view showing an example of apneumatic radial tire for a passenger car according to the presentinvention; and

FIG. 2 is a tire meridian half-sectional view showing another example ofthe pneumatic redial tire for a passenger car according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIGS. 1 and 2, reference numeral 1 denotes a tread portion, referencenumeral 2 denotes a bead portion and reference numeral 3 denotes asidewall portion. One carcass layer 4 formed by arranging reinforcingcords in a tire radial direction is disposed inside a tire. A ring-likebead core 5 is provided to each of the right and left bead portions 2and a ring-like bead filler 7 is so disposed as to continue from theouter periphery of each bead core 5. Both end portions 4A of the carcasslayer 4 are turned up from the inside to the outside of the tire aroundthe bead cores 5 in such a manner as to clamp bead fillers 7,respectively. Two layers of belt layers 6 are buried on the outerperipheral side of the carcass layer at the tread portion 1. This beltlayer 6 comprises an inner first belt layer 6A disposed adjacent to thecarcass layer 4 and a second belt layer 6B disposed outside the firstbelt layer 6A. The second belt layer 6B has a smaller width than thefirst belt layer 6A. These two belt layers 6 are substantially the sameas the belt layers of conventional tires and are designed to bear astrength corresponding to 0.35 N/mm·kPa in terms of a carcass strengthcoefficient. Symbol CL denotes a tire center line passing through a tireequatorial plane.

In the pneumatic tire having the construction described above, thecarcass layer 4 at the tread portion 1 has the carcass strengthcoefficient K defined by the afore-mentioned formula (1) which is withinthe range of 0.15 to 0.35 N/mm·kPa in a tread portion area on the tirecenter side of positions P of the tread portion 1, which are away by 10%of the belt width W, from both edges 6 b of the second belt layer (thebelt layer having the second greatest width) 6B, respectively towardinside. On the other hand, the carcass strength coefficient of thecarcass layer 4 so disposed as to extend to the bead portions 2continuing from the sidewall portions 3 from the positions P is set toat least 0.5 N/mm·kPa.

However, the number of carcass layers in the formula (1) is counted astwo layers at the portions where the carcass layer is turned up todouble. The term “maximum pneumatic pressure” represents the airpressure corresponding to the maximum load capacity stipulated by JATMA(The Japan Automobile Tire Manufacturers Association).

The carcass strength coefficient K can be made different in the tireradial direction as described above by extending both turn-up endportions 4A of the carcass layer 4 to the tread portion 1 so that theiredges 4 a reach the positions P as shown in FIG. 1. In other words, theportion of the tire from each position P to each sidewall portion 3 hasa double-layered structure in which the carcass layer 4 is turned up,while the tread portion 1 on the center side of each position P has asingle-layered structure. Therefore, when the carcass strengthcoefficient K of the carcass layer 4 disposed as a single layer is setto 0.25 to 0.35 N/mm kPa, the carcass strength coefficient on the sideof the sidewall portion from the position P of the tread portion 1becomes 0.5 to 0.7 N/mm kPa and can satisfy the range described above.

In place of the construction described above in which both turn-up endportions 4A of the carcass layer 4 are extended to the tread portion 1,it is also possible to employ the construction wherein at least onecarcass reinforcing layer 8 formed by arranging the reinforcing cords ina tire radial direction in the same way as the carcass layer 4 isdisposed outside the carcass layer 4 of each sidewall portion 3, asshown in FIG. 2. This carcass reinforcing layer 8 extends to the treadportion 1 so that its edge 8 a of its outer circumferential side 8Areaches the position P whereas the inner circumferential edge portion 8Bthereof extends to a portion on the inner side of each bead core 5 inthe tire radial direction. The carcass strength coefficient K can be setto the range described above by disposing the carcass reinforcing layer8 in this way, too.

Incidentally, the carcass strength coefficient K of the carcass layer 4on the sidewall portion from each position P is the sum of the carcassstrength coefficient K of the carcass layer 4 and the carcass strengthcoefficient K of the carcass reinforcing layer 8. The carcass strengthcoefficient K of the carcass reinforcing layer 8 can be obtained bysubstituting the numbers of the carcass reinforcing layers 8 for thenumber of the carcass layer in the formula (1).

The present invention can employ the combination of the constructionsshown in FIGS. 1 and 2, that is, the construction where both turn-up endportions 4A of the carcass layer 4 are extended to the points P plus theconstruction where the carcass reinforcing layers 8 are disposed outsideboth turn-up end portions 4A of the carcass layer 4.

In the present invention described above, the belt layers 6 share andbear a part of the tire internal pressure at the tread portion 1 wherethe belt layers 6 are disposed. Therefore, even when the strength of thecarcass layer 4 is lowered below that of the conventional tires, tiredurability does not get deteriorated. From this aspect, the carcassstrength coefficient K of the carcass layer 4 in the area correspondingto the belt layers can be set to the minimum value of 0.15 N/mm·kPawhich is smaller than the values of the conventional tires. Since thenumber of arranged reinforcing cords of the carcass layer 4 and thenumber of layers or the diameter of the reinforcing cords can be reducedin this way, the materials used can be drastically reduced, and theproduction cost can be reduced.

The carcass strength coefficient K is set to at least 0.5 N/mm·kPa inthe area in which the belt layers 6 are not disposed or speaking morecorrectly, in the carcass layer 4 between the position P and the beadportion 2, in the same way as in the conventional tires. As a result,tire durability can be kept at the same level as that of theconventional tires, and tire performance can be kept at the same levelas that of the conventional tires, too.

If the carcass strength coefficient K of a portion on the sidewall sideof the position P described above is smaller than 0.5 N/mm·kPa in thepresent invention, rigidity of the sidewall portion drops and tireperformance such as maneuvering stability gets deteriorated. The rangethat has been used in the past can be used as the upper limit withoutparticular limitation, but the value is preferably 1.5 N/mm·kPa for thereasons described above.

Any materials which have been used for the conventional tires can beused without particular limitation for the reinforcing cords of thecarcass layer 4. Organic fiber cords such as the polyester cords and thenylon cords can be used preferably. Similar materials to those of thecarcass layer 4 can be used for the reinforcing cords of the carcassreinforcing layer 8.

The number of arranged cords when the polyester cords are used for thereinforcing cords, for example, is 11 to 26 (pcs/50 mm) at a portion onthe center side of the tread portion of the position P in the case of1,000 d/2 cords and is 8 to 19 (pcs/50 mm) in the case of 1,500 d/2cords, and at a portion on the sidewall side of the position P, thenumber is at least 37 (pcs/50 mm) in the case of the 1,000 d/2 cords andis at least 27 (pcs/50 mm) in the case of the 1,500 d/2 cords at themaximum air pressure of 230 kPa.

When the nylon cords are used, the number of arranged cords at themaximum air pressure of 230 kPa is 12 to 27 (pcs/50 mm) at a portion onthe tread center side of the position P in the case of 840 d/2 cords andis 7.5 to 18 (pcs/50 mm) in the case of 1,260 d/2 cords, and the numberis at least 39 (pcs/50 mm) at a portion on the sidewall side of theposition P in the case of the 840 d/2 cords and is at least 25.5 (pcs/50mm) in the case of the 1,260 d/2 cords.

Incidentally, the term “number of arranged cords at a portion on thesidewall side of the position P” represents the sum of the number ofarranged reinforcing cords of the carcass layer 4 at the sidewallportion before turn-up and the number of arranged reinforcing cords ofthe carcass layer 4 on the turn-up side. In the case of the constructionwhere the carcass reinforcing layer 8 is disposed as shown in FIG. 2,the term represents the sum of the number of arranged reinforcing cordsof the carcass layer 4 and the number of arranged reinforcing cords ofthe carcass reinforcing layer 8.

In the embodiment of the present invention described above, both edges 4a of the carcass layer 4 or the edges 8 a of the outer circumferentialside edge portions 8A of the carcass reinforcing layer 8 are extended tothe positions P, but the edges 4 a and 8 a can be extended beyond thepositions P in such a manner as not to extend beyond the position P′which is away by 25% of the belt width W of the second belt layer 6Bfrom the tire center line CL outward in the tire width-wide direction,and the carcass strength coefficient K of the carcass layer in the areafrom at least the position P of the tread portion to the bead portionsmay be set to at least 0.5 N/mm kPa. The edges are preferably extendedsymmetrically but may be extended asymmetrically, too.

Though the embodiment given above represents the case where only onecarcass layer 4 is disposed, the number of the carcass layer 4 is notparticularly limited thereto and a plurality of carcass layers 4 may bedisposed, as well.

The belt layers 6, too, are not particularly limited to two layers, anda greater number of belt layers 6 may be disposed, as well. The presentinvention can be used preferably for a pneumatic radial tire equippedwith at least one carcass layer 4 and two belt layers 6, particularlyfor a pneumatic radial tire for passenger cars.

EXAMPLE

Tires according to the present invention (present tires) 1 to 9,comparative tires 1 to 4 and a prior art tire, each having a common tiresize of 165SR13 and having a construction tabulated in Table 1, wereproduced, respectively.

A durability evaluation test of a carcass layer of each of these testtires in a tread portion area on the tire center side of the position P(hereinafter called the “belt portion”) and in an area on the sidewallside of the position P (hereinafter called the “side portion”) wasconducted and the production cost was evaluated, under the followingmeasurement condition. The results shown in Table 1 were obtained.

Carcass durability

Each test tire was fitted to a rim having a rim size of 13×41/2-J andwas subjected to an indoor drum test at an air pressure of 230 kPa, aload of 9.3 kN, a speed of 80 km/hr and a room temperature of 38° C.After driving 5,000 km, the existence of breakage of the reinforcingcords of the carcass layer (inclusive of the carcass reinforcing layerwhen it was disposed) at the belt portion and the side portion wasmeasured. O represents that breakage did not exist and x represents thatbreakage occurred.

Production cost

The material cost of the carcass layer and the carcass reinforcing layerused for each test tire was calculated and the result was evaluated byan index value using the material cost of the prior art tire as 100. Thesmaller this value, the lower the production cost.

TABLE 1 present present present present present present present presentpresent tire 1 tire 2 tire 3 tire 4 tire 5 tire 6 tire 7 tire 8 tire 9structure FIG. 2 FIG. 2 FIG. 1 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 2 FIG. 1carcass cord material polyester polyester polyester polyester nylonnylon polyester polyester polyester layer cord spec. 1000d/2 1000d/21000d/2 1500d/2 840d/2 1260d/2 1000d/2 1000d/2 1000d/2 belt portion, thenumber of 11 18.5 18.5 13.5 19.5 13 25.5 15.5 18.5 arranged cords(pcs/50 mm) side portion, tbe number of 13 22 44 16 23.5 15.5 30 18.5 44arranged cords (pcs/50 mm) reinforcing cord material polyester polyester— polyester nylon nylon polyester polyester — layer cord spec. 1000d/21000d/2 — 1500d/2 840d/2 1260d/2 1000d/2 1000d/2 — number of arrangedcords 31 22 — 16 23.5 15.5 14 18.5 — (pcs/50 mm) carcass strengthcoefficient at belt 0.15 0.25 0.25 0.25 0.25 0.25 0.35 0.21 0.25 portion(N/mm · kPa) carcass strength coefficient at side 0.6 0.6 0.6 0.6 0.60.6 0.6 0.5 0.6 portion (N/mm · kPa) edge position (%) 0.1W 0.1W 0.1W0.1W 0.1W 0.1W 0.1W 0.1W 0.25W carcass durability at belt portion ∘ ∘ ∘∘ ∘ ∘ ∘ ∘ ∘ carcass durability at side portion ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ cost 9497 92 97 97 96 99 92 99 compara- compara- compara- compara- prior arttive tire 1 tive tire 2 tive tire 3 tive tire 4 tire structure FIG. 2FIG. 2 FIG. 2 FIG. 2 — carcass cord material polyester polyesterpolyester polyester polyester layer cord spec. 1000d/2 1000d/2 1000d/21000d/2 1000d/2 belt portion, the number of 7.5 29 18.5 25.5 375arranged cords (pcs/50 mm) side portion, tbe number of 9 34.5 22 30.544.5 arranged cords (pcs/50 mm) reinforcing cord material polyesterpolyester polyester polyester — layer cord spec. 1000d/2 1000d/2 1000d/21000d/2 — number of arranged cords 35 9.5 7 13.5 — (pcs/50 mm) carcassstrength coefficient at belt 0.1 0.4 0.25 0.35 0.51 portion (N/mm · kPa)carcass strength coefficient at side 0.6 0.6 0.4 0.6 0.61 portion (N/mm· kPa) edge position (%) 0.1W 0.1W 0.1W 0 — carcass durability at beltportion x ∘ ∘ x ∘ carcass durability at side portion ∘ ∘ x ∘ ∘ cost 93100 89 98 100 Remarks) {circle around (1)}The prior art tire had thestructure shown in FIG. 2, but without the carcass reinforcing layer.{circle around (2)}The difference between the numbers of arrangedreinforcing cords of the carcass layer at the belt portion and at theside portion in the pneumatic radial tire shown in FIG. 2 results fromthe increased diameter at the time of molding. {circle around (3)}Theedge position means positions of both edges of the carcass layer or theouter circumferential edge of the carcass reinforcing layer.

It can be understood clearly from Table 1 that the present tires canreduce the production cost by lowering the carcass strength coefficientat the belt portion and can keep carcass durability at the same level asthat of the prior art tire.

As described above, the present invention sets the carcass strengthcoefficient to a lower value of 0.35 N/mm·kPa at the tread portion, atwhich the belt layers bear a part of the tire internal pressure, than inthe prior art tires. Therefore, because the number of arrangedreinforcing cords of the carcass layer at the tread portion, thediameter of the reinforcing cords, the number of the carcass layers,etc, can be reduced, the materials used can be reduced. Accordingly, theproduction cost can be reduced.

In the areas from the positions P to the bead portions where the tireinternal pressure cannot be borne by the belt layers, the carcassstrength coefficient is set to at least 0.5 N/mm·kPa in the same way asin the prior art tires, while it is set to at least 0.15 N/mm·kPa at thetread portion. Consequently, required carcass durability can be securedand tire performance does not get deteriorated.

INDUSTRIAL APPLICABILITY

The present invention having the excellent effects described above canbe utilized extremely advantageously for pneumatic radial tires fittedto vehicles, particularly for pneumatic radial tires for passenger cars.

What is claimed is:
 1. A pneumatic radial tire wherein both end portionsof a carcass comprising at least one carcass layer formed by arrangingreinforcing cords are turned up around right and left bead cores fromthe inside to the outside of the tire, respectively, and at least twobelt layers formed by arranging reinforcing cords in such a fashion thattheir inclining directions with respect to a tire circumferentialdirection cross one another in mutually opposite directions between thelayers are disposed on the outer circumferential side of the carcass ofa tread portion, said at least two belt layers having a belt strengthcoefficient of at least 0.35 N/mm·kPa, said reinforcing cords of said atleast one carcass layer being selected from the group consisting ofpolyester cords and nylon cords, wherein a carcass strength coefficientK of said carcass, defined by the following formula, is set to 0.15 to0.35 N/mm·kPa for portions of the tread portion on the tread portioncenter side of positions P and is set to at least 0.5 N/mm·kPa atportions from said positions P to bead portions: K (N/mm·kPa)={number ofarranged reinforcing cords (pcs/mm)}×{strength of reinforcing cords(N)}×{number of carcass layers}÷(maximum air pressure (kPa)}, whereincarcass reinforcing layers formed by arranging reinforcing cords aredisposed outside said carcass layers at both sidewall portions,respectively, and said carcass strength coefficient K calculated as thesum of that of said carcass layers and that of said carcass reinforcinglayers is set to at least 0.5 N/mm·kPa, and wherein the end portion ofsaid carcass reinforcing layers on the outer circumferential sideextends to said positions P which are between positions away by 25% ofthe belt width of said belt layer having a second greatest width from atire center line toward the outside of the tire in the tire width-wisedirection and positions away by 10% of the belt width of said belt layerhaving a second greatest width from both edges thereof toward itsinside.